The invention relates to semiconductor opto-electronic devices and fabrication methods thereof, and more particularly, to white light emitting diodes and fabrication methods thereof.
Light emitting diodes (LEDs) are a special type of semiconductor diode first developed in the 1960s. The simplest LED comprises a p-type semiconductor and an n-type semiconductor forming a p-n junction. When an electric current passes through the junction, charge carriers (electrons and holes) are created. In the process, an electron combines with a hole and releases energy in the form of a photon. Most current high efficiency LEDs have one or more layers of light emitting materials sandwiched between the p- and n-type regions to improve the light emitting efficiency. The layer structure is also used to obtain desired emission wavelengths. A basic LED device comprises a small piece of such layered material, called a die, placed on a frame or a baseboard of electrical contacts and mechanical support. The die is also encapsulated for protection.
With an LED, the wavelength of the emitted light is determined by the bandgap energy of the light emitting material. A material suitable for LEDs is a compound semiconductor having bandgap energies corresponding to near infrared (IR), visible or near ultraviolet (UV) light. AlGaInP (Aluminum Gallium Indium Phosphide) is an LED material that exhibits high quantum efficiency (hence high brightness) and multiple colors. The bandgap of (AlxGa1-x)1-yInyP alloy system varies, depending on the x and y in the composition. The color of AlGaInP LEDs ranges from green to red. AlGaInP LEDs can be fabricated on a lattice-matching gallium arsenide (GaAs) substrate using an exitaxial growth process, such as the metalorganic chemical vapor deposition (MOCVD).
In the 1990s violet, blue and green LEDs based on gallium nitride (GaN) materials were developed. GaN is a direct bandgap semiconductor with bandgap energy of ˜3.4 eV. The electron-hole recombination in GaN leads to emission of photons at a wavelength of 360 nm, which is in the UV range. The visible wavelength LEDs (green, blue and red) are achieved by using InzGa1-zN as the light emitting layer, sandwiched between a p-type GaN layer and an n-type GaN layer. The wavelength λ of the light emitted by the InzGa1-zN system varies depending on the z value. For example, for pure blue color, λ=470 nm, z=0.2. The GaN LEDs must be fabricated on a lattice-matching substrate such as sapphire or silicon carbide (SiC), again using an epitaxial growth process such as MOCVD.
Great efforts have been made to produce white LEDs capable of replacing conventional lighting sources. Currently, white LEDs can be accomplished in various ways:
(1) Putting discrete red, green and blue LEDs in a “lamp” and use various optical components to mix light in red, green and blue colors emitted by those discrete LEDs. However, due to the different operating voltages for LEDs of different colors, multiple control circuits are required. Furthermore, the lifetime of the LEDs is different from one color to another. Over time the combined color would change noticeably if one of the LEDs fails or degrades.
(2) Partially converting light in short wavelengths to light in the longer wavelengths using phosphors. One of the most common ways is to dispose a yellowish phosphor powder around a blue InGaN LED chip. The phosphor powder is usually made of cerium doped yttrium aluminum garnet (YAG:Ce) crystal. Part of the blue light emitted by the InGaN LED chip is converted to yellow by the YAG:Ce. However, the produced “white” light contains mainly two colors: blue and yellow. Such a light source is usually used as an indicator lamp.
(3) Using UV light produced by very short-wavelength LEDs to excite phosphors of different colors in order to produce light of three colors. The drawback of this method is that the lifetime of the UV LEDs is relatively short. Furthermore, UV radiation from the LEDs can be a health hazard, as most commonly used encapsulation materials are not effective in blocking UV radiation.
There have been numerous attempts to develop white LED light sources with higher efficiency and better chromaticity. Guo et al. (“Photon-Recycling for High Brightness LEDs”, Compound Semiconductor 6(4) May/June 2000) discloses the concept of photon recycling in producing high brightness white LEDs. Photo recycling is a process by which short wavelength photons are absorbed by an emitter material, which re-emits photons for long wavelengths. In principle, photon recycling semiconductor (PRS) LEDs can efficiently produce white light of up to 330 lumen/watt. However, the drawback of PRS-LEDs is their extremely low color-rending index.
The dual-color PRS-LED, as disclosed in Guo et al., comprises a primary light source and a secondary light source. The secondary light source has a secondary light emitting layer. The primary light source is used to produce blue light. The produced blue light directed to the secondary emitting layer so that part of the blue light is absorbed in order to produce yellow light in the re-emitting process. In principle, the dual-color photon production in PRS-LEDs is analogous to the phosphor coated LED. However, unlike the phosphor coated LED, the secondary light source comprises a fluorescent semiconductor material, AlGaInP, directly bonded to the primary light source wafer. It is therefore possible to produce dual-color LED chips on a wafer.
FIG. 1 is a cross section of a conventional white light emitting diode device. A white LED device 100 comprises a blue-red dual-color LED 100a and a green-red dual-color LED 100a′. Both LEDs 100a and 100a′ are mounted on various electrically conductive sections 172, 174 and 176 on a baseboard 180. LEDs 100a comprise a primary light source 101B emitting blue light and a secondary light source 122a emitting red light which is converted from blue light.
The primary light source 101B comprises a first active layer 114, a hole source layer 112 to provide holes to the active layer 114, and an electron source 116 to provide electrons to the active layer 114 so that at least part of the electrons combine with at least part of the holes at the active layer 114 to produce blue light. The secondary light source 122a comprises an AlGaInP layer 122R absorbing part of the blue light of the primary light source 101B and re-emitting red light.
A p-type electric contact 132 of the blue-red color LED 100a electrically contacts the conductive section 172 on the baseboard 180 through conductive material 142. An n-type electric contact 136 of the blue-red color LED 100a electrically contacts the conductive section 174 on the baseboard 180 through conductive material 146. The configuration of the green-red color LED 100a′ is similar to the blue-red color LED 100a. Both the blue-red color LED 100a and the green-red color LED 100a′ are connected in series. Electrical contacts 152, 156 and wire bonds 162, 166 are further provided on the baseboard 180 to provide an electrical current through the serially connected LEDs.
The conventional white light LED device 100 is formed by wafer bonding. Wafer bonding, however, requires thinning and polishing the sapphire substrate causing high production cost. On the other hand, wafer bonding is an intricate process with production yield issues. Moreover, attaching red epitaxial layer on the GaN LED requires resolving electrode connection issues.